I think they should add the Hokkaido island/region to the map. The Matsumae clan was there after all. It would also be cool to have the Sakhalin region to explore/colonize.
Because one small clan settled there 120 years after the game's start date (and only 25 before the end) doesn't really warrant adding Hokkaido in my opinion. At least not as a playable area.
Hokkaido has already been discussed to some degree on thus forum. See these threads:
http://forum.paradoxplaza.com/forum/showthread.php?530407-Sengoku-Development-Diary-2-The-Map&highlight=hokkaido
http://forum.paradoxplaza.com/forum/showthread.php?524185-Map-Suggestion&highlight=hokkaido